Wildcats are wild cards that you can add to any set or streak as substitutes for any color or card type. You can also change the color of play on the discard pile. You can substitute wildcats as cover cards in a disguised set, although you can’t substitute them in a stack. You should only play with four wildcats in the game; any additional characters you obtain of any kind should be considered replacements.   

Wildcat sets

Wildcats can be made into a set by themselves with no special treatment.